PERF: couple of speed up improvements for QuadEdgeMesh processing

In QuadEdgeMeshBoundaryEdgesMeshFunction::Evaluate(), change a local
container from std::list<> to std::set<> to improve performance of removing
elements.

In QuadEdgeMesh, make use of GetElementIfIndexExists to avoid doing
too many ::find on the MapContainer.

Change-Id: Ic76674c64c3959f527e68b6c0ce9893deb0e1155
Merge topic 'SpeedUpMeshBoundary'

e5af98c PERF: couple of speed up improvements for QuadEdgeMesh processing